The TCA cycle, also known as the Krebs cycle or citric acid cycle, involves the oxidation of acetyl-CoA to carbon dioxide and water and occurs in the mitochondrial matrix. It is the final common pathway for carbohydrates, fats, and proteins and generates energy in the form of ATP, NADH, and FADH2. The cycle consists of 8 steps where citrate is regenerated at the end to continue the cycle. The cycle supplies energy and intermediates for biosynthesis and is regulated by substrate availability and product inhibition.

1. TCA Cycle

2. TCA Cycle
 Also known as Krebs cycle
 TCA cycle essentially involves the oxidation of
acetyl CoA to CO2 and H2O.
 TCA cycle –the central metabolic pathway
 The TCA cycle is the final common oxidative
pathway for carbohydrates, fats, amino acids.

3.  TCA cycle supplies energy & also provides many
intermediates required for the synthesis of amino
acids, glucose, heme etc.
 TCA cycle is the most important central pathway
connecting almost all the individual metabolic
pathways.

4.  Definition
 Citric acid cycle or TCA cycle or tricarboxylic acid
cycle essentially involves the oxidation of acetyl
CoA to CO2 & H2O.
 Location of the TCA cycle
 Reactions of occur in mitochondrial matrix, in
close proximity to the ETC.

5. Reactions of TCA cycle
 Oxidative decarboxylation of pyruvate to acetyl
CoA by PDH complex.
 This step is connecting link between glycolysis and
TCA cycle.

6. Reactions of TCA Cycle
 Step:1 Formation of citrate
 Oxaloacetate condenses with acetyl CoA to form
Citrate, catalysed by the enzyme citrate synthase
 Inhibited by:
 ATP, NADH, Citrate - competitive inhibitor of
oxaloacetate.

7. Steps 2 & 3 Citrate is isomerized to isocitrate
 Citrate is isomerized to isocitrate by the enzyme
aconitase
 This is achieved in a two stage reaction of
dehydration followed by hydration through the
formation of an intermediate -cis-aconiase

8. Steps 4 & 5 Formation of -ketoglutarate
 Isocitrate dehydrogenase (ICDH) catalyses the
conversion of (oxidative decarboxylation) of isocitrate
to oxalosuccinate & then to -ketoglutarate.
 The formation of NADH & the liberation of CO2
occure at this stage.
 Stimulated (cooperative) by isocitrate, NAD+, Mg2+,
ADP, Ca2+ (links with contraction).
 Inhibited by NADH & ATP

9. Step: 6 Conversion of -ketoglutarate to
succinyl CoA
 Occurs through oxidative decarboxylation,
catalysed by -ketoglutarate dehydrogenase
complex.
 -ketoglutarate dehydrogenase is an multienzyme
complex.
 At this stage of TCA cycle, second NADH is
produced & the second CO2 is liberated.

10. Step: 7 Formation of succinate
 Succinyl CoA is converted to succinate by
succinate thiokinase.
 This reaction is coupled with the phosphorylation
of GDP to GTP.
 This is a substrate level phosphorylation.
 GTP is converted to ATP by the enzyme nucleoside
diphosphate kinase.

11. Step: 8 Conversion of succinate to fumarate
 Succinate is oxidized by succinate dehydrogenase
to fumarate.
 This reaction results in the production of FADH2.
 Step: 9 Formation of malate: The enzyme
fumarase catalyses the conversion of fumarate to
malate with the addition of H2O.

12. Step:10 Conversion of malate to
oxaloacetate
 Malate is then oxidized to oxaloacetate by malate
dehydrogenase.
 The third & final synthesis of NADH occurs at this
stage.
 The oxaloacetate is regenerated which can
combine with another molecule of acetyl CoA &
continue the cycle.

13. Pyruvate
Acetyl CoA
Citrate
Cis-Aconitase
Iso-citrate
Oxalosuccinate
ɑ-Ketoglutarate
Succinyl CoA
Succinate
Fumarate
Malate
Oxaloacatete
PDH
CO2, NADH + H+
NAD+
NADH + H+
NAD+
CO2, NADH + H+
NAD+
GDP+Pi
GTP
FADH2
FAD
- H2O
NADH + H+
NAD+
Citrate
synthase
Aconitase
Aconitase
SDH
Fumarase
TCA

14. From: Summerlin LR (1981) Chemistry for the Life Sciences. New York: Random House p 550.

15. Regeneration of oxaloacetate
 The TCA cycle basically involves the oxidation of
acetyl CoA to CO2 with the simultaneous
regeneration of oxaloacetate.
 There is no net consumption of oxaloacetate or any
other intermediate in the cycle.

16. Significance of TCA cycle
 Complete oxidation of acetyl CoA.
 ATP generation.
 Final common oxidative pathway.
 Integration of major metabolic pathways.
 Fat is burned on the wick of carbohydrates.
 Excess carbohydrates are converted as neutral fat
 No net synthesis of carbohydrates from fat.
 Carbon skeleton of amino acids finally enter the TCA cycle.

17. Requirement of O2 by TCA cycle
 There is no direct participation of O2 in TCA cycle.
 Operates only under aerobic conditions.
 This is due to, NAD+ & FAD required for the
operation of the cycle can be regenerated in the
respiratory chain only in presence of O2.
 Therefore, citric acid cycle is strictly aerobic.

18. Energetics of TCA Cycle
 Oxidation of 3 NADH by ETC coupled with
oxidative phosphorylation results in the synthesis of
9ATP.
 FADH2 leads to the formation of 2ATP.
 One substrate level phosphorylation.
 Thus, a total of 12 ATP are produced from one
acetyl CoA.

21. Regulation of TCA Cycle
 Three regulatory enzymes
1. Citrate synthase
2. Isocitrate dehydrogenase
3.α-ketoglutarate dehydrogenase

22.  Citrate synthase is inhibited by ATP, NADH, acyl
CoA & succinyl CoA.
 Isocitrate dehydrogenase is activated by ADP &
inhibited by ATP and NADH
 α-ketoglutarate dehydrogenase is inhibited by
succinyl CoA & NADH.
 Availability of ADP is very important for TCA
cycle to proceed.

23. Inhibitors of TCA Cycle
 Aconitase is inhibited by fluoro-acetate.
 This is a non-competitive inhibition.
 Alpha ketoglutarate is inhibited by Arsenite.
 This is also a non-competitive.
 Succinate dehydrogenase is inhibited by malonate.
 This is competitive inhibition.

24. Amphibolic nature of the TCA cycle
 TCA cycle is both catabolic & anabolic in nature,
called as amphibolic.
 Since various compounds enter into or leave from
TCA cycle, it is sometimes called as metabolic traffic
circle.

25. Important anabolic reactions of TCA cycle
 Oxaloacetate is precursor for aspartate.
 α-ketoglutarate can be transaminated to
glutamate.
 Succinyl CoA is used for synthesis of heme.
 Mitochondrial citrate is transported to cytoplasm
& it is cleaved into acetyl CoA to provide
substrate for fatty acid synthesis.

26. Anaplerosis or anaplerotic reactions
 The reactions concerned to replenish or to fill up
the intermediates of citric acid cycle are called
anaplerotic reactions or Anaplerosis

27. Important anaplerotic reactions
 Pyruvate carboxylase catalyses conversion of
pyruvate to oxaloacetate.
 This is an ATP dependent carboxylation reaction.
Pyruvate+CO2+ATP Oxaloacetate + ADP + Pi

28. • Pyruvate is converted to malate by NADP+
dependent malate dehydrogenase (malic enzyme).
Pyruvate + CO2 + NADPH + H+
malate + NADPH + H2O

29.  α- ketoglutarate can also be synthesized from
glutamate by glutamate dehydrogenase.
Glutamate + NAD(P) + H2O α-
ketoglutarate +NAD(P)H + H+ + NH4
+

30. Transamination
 Transamination is a process where an amino acid
transfers its amino group to a keto group and itself
gets converted to a keto acid.
 The formation of Alpha ketoglutarate &
oxaloacetate occures by this mechanism.

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